Tumors with mutant BRAF or mutant KRAS are dependent on ERK signaling and sensitive to MEK inhibitors, but only the BRAF mutant tumors are sensifive to RAF inhibitors. We have shown that RAF inhibitors allosterically activate RAS-dependent F?AF dimers in most normal and tumor cells and thus paradoxically acfivate signaling. However, in tumors with V600E BRAF mutafion, activated ERK causes feedback inhibition of RAS.GTP to a levels too low to support RAF dimerization and in this context V600E signals as a monomer. RAF inhibitors bind to the monomer and potently inhibit ERK signaling in these tumors. This is basis for the dramafic therapeutic response of melanomas with codon 600 BRAF mutafion to these drugs compared to MEK inhibitors. However, tumor responses are incomplete and often temporary. Tumor progression is due to acquired resistance often characterized by insensitivity of ERK signaling to the RAF inhibitor. We have shown that this may be mediated by inducfion of RAS.GTP or to splice variants of RAF that dimerize in a Ras-independent manner. We also believe that the inifial tumor response is limited by adaptafion of the tumor to inhibifion of ERK. In RAS and BRAF mutant tumors, ERK acfivation causes the feedback inhibition of other intracellular signaling pathways and renders the cell dependent on ERK signaling. This causes hypersensitivity to RAF inhibitors (mutant BRAF tumors) or MEK inhibitors (mutant BRAF and some KRAS tumors). However, inhibition of ERK signaling with these drugs relieves this feedback, attenuates inhibifion of ERK output, and acfivates other mitogenic signaling pathway that cause adaptive resistance to ERK inhibifion. Our goals in this proposal are to develop therapies that maximally inhibit ERK output by combining RAF or MEK inhibitors with selective MEK and RTK inhibitors that prevent feedback reactivation of RAF. We hypothesize that maximal inhibition of ERK output with these regimens will relieve feedback inhibition of receptor tyrosine kinase signaling and cause resistance in that manner. We will identify these reactivated pathways and then develop and test therapies based on maximal ERK inhibifion combined with inhibifion of key reacfivated receptors to prevent or limit adaptive resistance.